Method and composition for forming a porous metallic coating



METHOD AND COMPOSITION FOR FORMING A POROUS METALLIC COATING William S. Bowles, Mount Royal, Montreal, Quebec,

Canada, assignor to Wall Colmonoy Corporation, Detroit, Micln, a corporation of New York No Drawing. Application October 12; 195 Serial No. 615,491

16 Claims. 01. 117+22 I This invention relates to improved metallic coating compositions for application by spraying to metal or other surfaces and to improved methods of forming metallic surface coatings.

Spray coating of metallic compositions "on metal surfaces is Well known to build up such surfaces to a desired dimension, to increase their corrosion resistance, and to improve their wear and abrasion resistance. Such coat ings may be applied by spraying metal in molten form upon the-surface tobe treated, the individualdroplets of the metal congealing as theystrike the surface, and

fusing together to build up a relatively thick, continuous coating. The metal to be sprayed may, alternatively, be

sprayed at a temperature below its melting point, In this case, the metal is first powdered, and is heated during the spraying process to a temperature slightly below its melting point at which temperature it is relatively plastic. In this case, the individual particles of the sprayed metal do not weld together to form a continuous coating, but being plastic, deform when they impinge upon the surface and interlock with each other, and with irregularities in the surface to form a mechanically bonded coating. Suchcoatings are relatively porous and are advantageous for many uses, particularly since they are capable of retaining relatively large quantities of" lubricants. v

The present invention pertains especially to improved compositions particularly adapted for application by the latter described-method. The compo-sitionsof the invention are especially advantageous for surface coating. of metallic and other surfaces to improve theirfwear and corrosion resistance, their capacity to retain lubricants, and to build them up dimensionally.

Accordingly, one important object of the present invention is to provide improved composition for metal spraying.

Another object is to provide improved metal spraying compositions which when applied. by spraying at temperatures below their melting temperatures, produce surface coatings which are readily machinable and highly resistant to abrasion, corrosion and wear.

Another object is to provide improved metal spraying compositions for producing relatively porous, highly wear and corrosion resistant and yet readily machinable coatings. H 7

Still another object is to provide improved methods-of forming porous, metallicsurface coatings.

These and other objects are. accomplished according to the present invention, the improved composition of which comprise mixtures of chromium-nickel-boron'alloys with relatively soft metals such as iron, cobalt and nickel, and alloys based upon iron, nickel and cobalt. The

2,858,667 Patented Jan. 13, 1959 one of the ingredientsis plastic, but below the temperature at which either of the ingredients melts. The sur-. face coating resulting from such spraying is relatively porous and is capable of retaining relatively large quantities of a lubricant. :The chromium-nickel-boron alloy lends hardness to the coating and provides weariand abrasion resistance closely approximating the Wear and abrasion resistance achieved by a similarly sprayed coating of the chromium nickel-boro-n alloy alone. The soft metal improves the machining and grinding characteris-= tics of the coating without significantly decreasing its. More-- over, sinceflthe softer metals are relatively inexpensive compared to thechromium-nickel-boron alloy, the=com'- positions of the invention provide metallic surface coat:

wear resistance in metal to metal applications.

ings of high quality at reduced cost.

It should also be noted that the hard chromium-nickel boron alloys are non-heat treatable, and will retain their. hardness despite repeated heating up to their melting temperatures and cooling in any manner;

The chromium-nickel-boron' alloys that are preferred for use in the practice of the invention are those falling within the following analyses: Percent Carbon; 0.30 to 1.00 Chro-mium 8.00 to 18.00 Nickel"; 65.00 to 85.00 Boron V I 2.00 to 4.50 I-ron i 1.25; 5.50 Silicon 1.25 1'to 5.50 Cobalt Up to 1.50

These alloys arerelativ ely hard, and are difficult to finish by machining or grinding. They can'zbe machined only with great difiiculty, using metallic carbide and aluminum oxide cutting tools. and slow procedure when dealing with these; alloys. Previously, these alloys have been widely used in a process called spray welding in which thepowdered alloy is sprayedinplastic'condition upon a surface to be. coated, and subsequently fused on the surface by heating to a temperature above its melting temperature. Spray weldingis advantageous in many instances, particularly where it is desired to achieve a non-porous surface coatingof ,These :alloys may also be" relatively great hardness. sprayed in plastic form upon a surface to producea porous coating in like manner to the spraying of the compositions according to the invention. Such porous coatings comprising *only the chromiumTnickel-boron alloys, however, are relatively hard and difficult to finish. They are also relatively expensive.

According to the present invention, these two disadvantages are overcome by the admixture of a relatively itself corrosion resistant and may be, for example, stainmixtures of the invention are made up in the solid state, i

a surface to be treated at a temperature at which at least" softmetal to the chromium-nickel-boron alloy. The soft metal may be added to the hard alloy in proportions of about. 20% to by weight based on'the, total weight of the mixture. The identity ofthe soft metal isnot generally critical in the practice of the invention, but may. be determined by the specific properties desired in the coating. For example, if a high degree of corrosion resistance is desired, therelativelysoft metal should be less steel, or an alloy high in nickel or cobalt. If ready machinability-is of importance in the coating, a readily machinable soft metal should be selected such as, for example, a soft steel.

Certain'metals which are readily oxidizable or which form refractory oxides such as aluminum, magnesium, titanium, tungsten, and chromium cannot be readily sprayed under normal conditions. They maybe used in I the practice of the invention,'howeve'r,- if the proper process conditions are provided, principally with respect t Even grinding is a relatively difficult controlling the atmosphere surrounding the metals during spraying. In general, for most co-mmercial'applicatrons it is believed that iron, cobalt, and nickel, and relatively soft alloys including iron, cobalt, or nickel as their principal ingredient will be found -'m ost-a"dvaritageous for admixture with the chromium-hickehboron al loys according to the invention. r Y According to a preferred embodiment of-the invention, achromium-nickel-boron alloy of the following composi-1 tion is prepared: 10

Percent" Carbon 0.50 to 1.00} Chromium 12.00 to 18.00 Nickelnna 65.00 t0 7 5.00 Bor n 2.50 to'4.50 Iron 3.50 to 5.50 Silicon 3.50 to 5.50 Cobalt 1.00 maximum.

This alloy' may be prepared in a conventional manner. It is pulverized by anyconvenient means, preferably by crushing, for a sufiicient time to reduce it to a relatively fine'powder. The size of the individual powder particles is not critical in the practice of the invention, but will be determined principally'by the type of apparatus used in the spraying process? For most commercially available apparatus adapted t'o's'pray hot metal powders, a particle size distribution of 100% throughan 80 mesh screen, 99%"through a 120 mesh screen, and less than 45% through a 325 mesh screen gives satisfactory results. The lower limitof size insures good flow characteristics of the powder and minimizes its tendency to compact and clog.

-Althoughany convenient method of'pulveri zing may be used lto prepare the metals for spraying according to the inyentionfci'ushiii'g in a'mill'is preferredsince it tends to produce: angular particles which more readily absorb heat" than spheroidal particles and are more readilylocked iniplace byaimechani'cal grippingaction. Particles formed by-atornizatioh of="t-he molten "metal with a gas su'chf'as" steam or airtend to be relatively rounded,'or spheroidal" 40 in shape."" Such'p'articles are relatively diflicultto hold in' a mechanically bonded coating, and also are relatively difficult to heat since their surface area is relatively small.

A quantity of 18-8 stainless steel is also pulverized to substantially the same particle size as the chromiumnickel-boron alloy. Approximately equal proportions by J Weight of the chromium-nickel-boron alloy and stainless 1 steel are physically mixed together by any desired means to produce an homogeneous, uniform mixture. This mixture is then propelled, preferably by means of a carrier gas such as air through aheating medium such as an oxy- 5U acetylene flame, and directed upon a surface to be coated. The surface to be coated is'preferably prepared by chilled iron grit blasting to roughen' it, so that the sprayed metal powders may interlock with thesurface-irregularities and bond thereto by'mechanical'gripping; l

The heating medium may' be, for example, an oxy-- acetylene torch operated with a'so called neutral flame. Its temperature is adjustedfa'nd its distance 'fromthe surface to be coated is-'arr ang'ed-'--sotha t at the time the metal particles ofthe powder strike the surface theyare heated U0 tojabout 1850? Atthis'tempe'ratur'e thepartiele'sof the chromium-nickel-bo'ron alloy areplastic'and are able todeform and to interlock with theothermetal particles and with the surface irregularities. Satisfactory spraying conditions m y be achieved using a torch having an 8- 5 hole tip, each hole being inch in diameter, with oxygen supplied at about 20 p. s. i., acetylene at about 12 to 15 p. s. i., and the torch being supported about 5 /2 "to6- inches from the surface being coated. The powder mix ture'is'preferably directed intothe flame near thetip of the torch, and is carried to the surface by the burning gas mixture of the flame and a propellant gas stream. The propellant gas stream maybe of air, and may be med by. d ti air: at.a.rressu.re.c .;abq 32.1949 p. s. {through a set of auxiliary holes, eight innumlier cool without'quenching, and may then be finished by machining or'grinding to a desired dimension. The coatingthus produced is machinable and may be easily ground Its machining and grinding characteristics are determined principally by the relatively soft metal included in the powder mixture It will be appreciated, however, that the machinability and grinding characteristics of the coating are affected to at least some extent by the chromium-nickel-boron alloy, so that those mixtures including relatively high proportions of the chro' mium-nickel-boron alloys are slightly more diflicult to machine and slower to grind than coatings including relatively' small proportions of the chromium-nickel-boron alloys:-

Coatings formed according to the preferred embodiment of the invention are machinable and may be readily ground. 'They have. excellent corrosion resistance properties and are highly resistant to wear and abrasion. Their wear resistance properties closely approach the wear resistance of surface coatings comprising chromium-nickelb oron alloys alone, yet their cost is substantially less, both because of reduced material cost (stainless steel is generally'much cheaper than chromium-nickel-boron alloys) and through a reduction in the cost of finishing the coatings to their final dimensions.

The spraying temperatures of the powder mixtures according to theinvention are relatively critical to achieve 5 satisfactory results. On the one hand, at least one of the powder ingredients should be in a plastic state so that its particles can be deformed upon impact to provide an interlocking mechanical bond to secure the coating to the 'treated surface and a metallurgical bond between the 1 particles of the coating. On the other hand, melting is to be avoided in order to preserve the oil retaining porosity of the coatingand the hardness and wear resistance of the chromium-nickel-boron alloy particles.

Equivalent results are not achieved if the powders are pro-alloyed, i. e., if the chromium-nickel-boron alloy is first alloyed by melting with the soft metal and then reduced to powder form and sprayed. Coatings made of such pre-alloys lack the wear resistance of coatings made according to the present invention, because they do not include the discreet, extremely hard particles of the chrominum-nickel-boron alloys, which particles appear to be largely responsible for the excellent wear resistance characteristics of coatings according to'the invention. For this reason, the spraying temperature is preferably controlled so 'that the individual powder particles are not molten at the moment of impact so that volume alloyingis avoided during the spraying process. The hard alloy particles thus retain their separate identity in the surface coating, and

significant degree.

"The particles of atleast one,- and preferably of bothof the metals are highly plastic at the moment of impact so that they wet the other particles and become metallurgically bonded, or'surface alloyed thereto, and thus produce a strongly cohesive coating.

The soft metal must be one that is capable of wetting or of being wetted by the hard alloy under these conditions, that is, it must be capable of forming a metallurgical bond with the hard alloy at a temperature below the melting point (liquidus) of the lower melting'one of the soft metal and the hard alloy.

In those cases where iron, nickel, or cobalt, or alloys based on iron, nickel, or. cobalt are used as the soft metal ingredient itwill usually be found that the chromiumaccordingly control the choice of aspraying temperature. The chromium-nickel boronv alloys preferredv for use. in the practice ofthe invention begin to soften at about 175 F.,-.a nd melt at about 1900 F. The upper limit of the sprayingtemperature (temperature of the powder atthe time of impact upon thesurface) is thereforeabout 1900 F., and in practical operation the actual temperature is maintained as close tothis temperature" as prudence and the nature of the soft metal ingredient permit. Iffthe soft'metalhas a higher melting point thanthe chromiumnickel-bo-ron alloy, a temperature of about 1850 F. is used. If the soft metal melts at temperatures below 1850 F. then a lower temperature is preferred, say-about 25 to 50 F. below the soft metalmelting, temperature.

There have thus beendescribed'improved metallic coating compositions and methods of spray coating surfaces to protect them against'wear, corrosion and abrasion, and to build up undersized parts. Surface coatings according to the invention-areporous and highly wear resistant, and their other properties such as'corrosion'resistance and machinability may be controlled within a broad range of variation.

What is claimed is: v

1. A metal spraying composition comprising 'anintimate mixture in powder formof a hard alloy consisting essentially of chromium, nickel, and boron, and a metal that is relatively soft compared to said alloy, said soft metal being one that is capable of forming a metallurgical bond with said hard alloy at a temperature" below the melting point of the lower melting one of 'said'hard alloy and said soft metal, whereby when said composition is spray applied to a workpiece at a temperature within-the plastic range but below the liquidus temperature of the lower melting one of said hard alloy and said soft metal the. individual particles become metallurgically bonded together to form a cohesive coating .in. whichthe hard alloy particles retain their identity.

2. A metal spraying composition consisting essentially of an;:-intirnate mixture in powder form of a hard alloy of chromium, nickel, and boron and a metal that is softer than said alloy, said soft metal comprising from 20% to 70% by weight of said mixtureand being one that is capable of forming a metallurgical bond with said hard alloy ata temperature below the melting point of the lower melting one of said hard alloy and said soft metal, whereby when said composition is spray applied to a workpiece at'a temperature within the plastic range but below the liquidus temperature of the lower melting one of said hard alloy and said soft metal the individual particles become metallurgically bonded together to form a cohesive coating in which the hard allo-y particles retain their identity.

3. A metal spraying compositionconsi'sting essentially of a mixture of a hard alloy and a soft metal, said alloy and said metal being intermixed in powder form, said hard alloy consisting essentially of:

Percent CarbOH t0 Chromium 8.00 to 18.00 Nickel 65.00 to 85.00 Boron 2.00, to 4.50

Iron 1.25 to 5.50 Silicon 1.25 to 5.50 Cobalt Up to 1.50

said soft metal being selected from the group consisting of iron, cobalt, nickel, and alloys based on at least one of iron, cobalt, and nickel, whereby when said composition is spray applied to a workpiece at a temperature within the plastic range but below the liquidustemperature of the lower melting one of said hard alloy and said soft metal the individual particles become metallurgically bonded together to form a cohesive coating in which the hard alloy particle's retain their identity.

4. A metal spraying composition consisting essentially of amixture-ofa hard alloy and 18-8stainless steel, said alloy and saidist'ainless steellbeing intermixed in'powder form, and said"hardalloy consisting essentially ofi Percent Carbon.- p a 1 I 0.50 to 1.00 Chromium 12.00 to 18.00 Nickel: 65.00 to 75.00 Boron"' 2.50 to 4.50 Iron" 1 I 3.50to.5.50 Silicon. .a 3.5 0 .to' 5 .50 Cobalt 7 Up to 1.00

whereby"whensaid'composition is spray applied to a Workpie'ceata temperature within the plastic range of saidihard alloy the'individual particles become metallurgically'bonded togetherto'form a cohesive coating, and the hard alloy particles retain their separatefidentity inthe coating.

5. Ametal spraying composition consisting essentially of a mixture of a hard alloyand a soft metaLsaid alloy and said metal being intermixed in powder form, said hard alloy consisting essentially of:

Percent Carbon 0.50 to 1.00 Chromium 12.00 to 18.00 Nickel 65.00to 75.00 Boron 2.50-to 4.50 Iron 3.50 to 5.50 Silicon 3.50 to 5.50 Cobalt Up to 1.00

1 and 70% to 20% by weight of a soft metal, said alloy and said metal being intermixed in powder form, said hard alloy consisting essentially of Percent Carbon 0.30 to 1.00 Chromium 8.00 to 18.00 Nickel 65.00 to 85.00 Boron 2.00 to 4.50

Iron Y 1.25 to5.50 Silicon 1.25 to 5.50 Cobalt Up to 1.50

said soft metal being selected from the group consisting of iron, cobalt, nickel, and alloys based on at least one of iron, cobalt, and nickel, whereby when said composition-is'spray applied to a workpiece at a temperature within the plastic range but below the liquidus temperature of the lower'meltingone of said'hard alloy and said soft metal the individual particles become metallurgically bonded together to form a cohesive coating inwhich the hard alloy particlesretain their identity.

7. A metal spraying composition consisting essentially of an equal weight proportion mixture of a hard alloy and 18-8stainless steel, said alloy and said stainless steel being intermixed in powder form, and said hard alloy consisting essentially of:

Cobalt I Up to 1.09

coating.

8. A metal spraying composition consisting essentially of an intimate mixture in powder form of an alloy of chromium, nickel, and boronand a metal that is softer than said alloy, said soft metal comprising from 20% to 70% by weight of said mixture and being one that is capable of forming a metallurgical bond with said hard alloy at a temperature below the melting point of the lower melting one of said hard alloy and said soft metal, said mixture having a particle size distribution of 100% through an 80 mesh screen, 99% through a 120 mesh screen, and less than 45% through a 325 mesh screen, whereby when said composition is spray applied to a workpiece at a temperature within the plastic range but below the liquidus temperature of the lower'melting one of said hard alloy and said soft metal the individual particles become metallurgically bonded together to form a cohesive coating in which the hard alloy particles retain their identity.

9. A metal spraying composition consisting essentially of a mixture of a hard alloy and a soft metal, said alloy and said metal being intermixed in powder form, said hard alloy consisting essentially of:

said soft metal being selected from the group consisting of iron, cobalt, nickel, and alloys based on at least one of iron, cobalt, and nickel, said mixture having a particle size distribution of 100% through an 80 mesh screen, 99% through a 120 mesh screen, and less than 45% through a 325 mesh screen, whereby when said composition is spray applied to a workpiece at a temperature within the plastic range but below the liquidus temperature of the lower melting one of said hard alloy and said soft metal the individual particles become metallurgically bonded together to form a cohesive coating in which the hard alloy particles retain their identity.

10. A metal spraying composition consisting essentially of an equal weight proportion mixture of a hard alloy and l88 stainless steel, said alloy and said stainless steel being intermixed in powder form, and said hard alloy consisting essentially of:

said mixture having a particle size distribution of 100% through an 80 mesh screen, 99% through a 120 mesh screen, and less than 45% through a 325 mesh screen,

whereby when said composition is spray applied to a workpiece at a temperature within the plastic range of said hard alloy the individual particles become metallurgically bonded together to form a cohesive coating and the hard alloy particles retain their separate identity in the coating. 11. Method of forming a porous metallic surface coat ing comprising the step of spraying upon a surface to be coated a'mixture in powder form of a hard alloy of chromium, nickel, and boron and a metal that is relatively soft compared to said alloy, at an elevated temperature at which at least one of said metal and said alloy is plastic but below the melting temperatures of said alloy and said metal, said soft metal being capable of forming a metallurgical bond with said hard alloy at said temperature, thereby to form a wear resistant and machinable cohesive surface coating in which the particles of said hard alloy retain their identity and are metallurgically bonded to said soft metal.

12. Method of forming a porous metallic surface coating comprising the step of spraying upon a surface to be coated a mixture in powder form of 30% to by weight of a hard alloy of chromium, nickel, and boron and 70% to 20% by weight of a metal that is relatively soft compared to said alloy, at an elevated temperature at which at least one of said metal and said alloy is plastic but below the melting temperatures of said alloy and said metal, said soft metal being capable of forming a metallurgical bond with said hard alloy at said temperature, thereby to form a wear resistant and machinable cohesive surface coating in which the particles of said hard alloy retain their identity and are metallurgically bonded to said soft metal. 1

13. Method of forming a porous metallic surface coating comprising the steps of spraying upon a surface to be coated a mixture in powder form of a hard alloy and a soft metal, said hard alloy consisting essentially of:

Percent Carbon 0.30 to 1.00

Chromium 8.00 to 18.00 Nickel 65.00 to 85.00 Boron 2.00 to 4.50

Iron 1.25 to 5.50 Silicon 1.25 to 5.50 Cobalt Up to 1.50

said soft metal being selected from the group consisting of iron, cobalt, nickel, and alloys based on at least one of iron, cobalt, and nickel, and heating said mixture so that it is at a temperature of about 1750 F. to 1900 F. when it impinges upon said surface, thereby to form a wear resistant and machinable cohesive surface coating in which the particles of said hard alloy retain their identity and are metallurgically bonded to said soft metal.

14. Method of forming a porous metallic surface coating comprising the steps of spraying upon a surface to be coated a mixture in powder form of a hard alloy and a soft metal, said hard alloy consisting essentially of:

said soft metal being selected from the group consisting of iron, cobalt, nickel, and alloys based on at least one of iron, cobalt, and nickel, and heating said mixture during said spraying to a temperature of about 1850 F., thereby to form a wear resistant and machinable cohesive surface coating in which the particles of said hard alloy retain their identity and are metallurgically bonded to said soft metal.

15. Method of forming a porous metallic surface coating comprising the steps of spraying upon a surface to be coated a mixture in powder form of 30% to 80% by weight of a hard alloy and 70% to 20% by weight of a soft metal, said alloy consisting essentially of:

- Percent Carbon 0.30 to 1.00 Chromium 8.00 to 18.00 Nickel 65.00 to 85.00 Boron 2.00 to 4.50 Iron 1.25 to 5.50 Silicon 1.25 to 5.50 Cobalt Up to 1.50

said soft metal being selected from the group consisting of iron, cobalt, nickel, and alloys based on at least one of iron, cobalt, nickel, and heating said mixture while it is being sprayed-to a temperature of about 1750 F. to

1900 F., thereby to form a wear resistant and machinable' cohesive surface coating in which the particles of said hard alloy retain their identity and are metallurgically bonded to said soft metal.

16. Method of forming a porous metallic surface coating comprising the steps of spraying upon a surface to be coated an equal weight proportion mixture in powder form of a hard alloy and 18-8 stainless steel, said hard alloy consisting essentially of:

and heating said mixture during said spraying to a temperature of about 1750 F. to 1900 F., thereby to form a wear resistant and machinable cohesive surface coating in which the particles of said hard alloy retain their identity and are metallurgically bonded to said-stainless steel.

References Cited in the file of this patent UNITED STATES PATENTS 1,091,057 Gilson Mar. 24, 1914 1,128,059 Schoop Oct. 16, 1934 2,261,228 Cockrum Nov. 4, 1941 2,744,009 Bowne et a1 May 1, 1956 OTHER REFERENCES Rose et al.: The Welding Journal, vol. 34, No. 1, page .30 relied upon. 

11. METHOD OF FORMING A POROUS METALLIC SURFACE COATING COMPRISING THE STEP OF SPRAYING UPON A SURFACE TO BE COATED A MIXTURE IN POWDER FORM OF A HARD ALLOY OF CHROMIUM, NICKEL, AND BORON AND A METAL THAT IS RELATIVELY SOFT COMPARED TO SAID ALLOY, AT AN ELEVATED TEMPERATURE AT WHICH AT LEAST ONE OF SAID METAL AND SAID ALLOY IS PLASTIC BUT BELOW THE MELTING TEMPERATURES OF SAID ALLOY AND SAID METAL, SAID SOFT METAL BEING CAPABLE OF FORMING A METALLURGICAL BOND WITH SAID HARD ALLOY AT SAID TEMPERATURE, THEREBY TO FORM A WEAR RESISTANT AND MACHINABLE COHESIVE SURFACE COATING IN WHICH THE PARTICLES OF SAID HARD ALLOY RETAIN THEIR IDENTITY AND ARE METALLURGICALLY BONDED TO SAID SOFT METAL. 